A radio receiver suitable for use in a spaced carrier area coverage system. The radio receiver may be used with an area coverage system for land mobiles and in a conceivable maritime area coverage system.
Spaced carrier area coverage systems are used where it is desired to transmit a signal to, say, a vehicle moving within an area and which signal, if transmitted from a single aerial, could be subject to extensive fading or loss at the receiver. In a known amplitude modulated spaced carrier area coverage system, typically three aerials are located at vantage points within the area to be covered. Each aerial is associated with its own transmitter which transmits a signal on its respective carrier wave. The frequencies of these carrier waves are chosen so that all the signals including their sidebands fall substantially within a 25 kHz channel. The narrow bandwidth modulation signals have to be synchronized so that they are all in phase. The selection of the frequencies of the carrier waves is such that the primary, secondary and subsequent beat notes fall outside the audio bandwidth, typically 200 Hz to 3 kHz, of the receiver.
In order to receive the signal, the known receiver has an IF noise bandwidth (that is the passband between the .+-.3 dB points) of typically 20 kHz so that it is able to receive all three carrier signals and at least one sideband of each carrier. Because of this wide bandwidth the receiver picks up unwanted noise. Trying to select any one of these carriers by having say three channel filters each with a response of .+-.3.75 kHz, instead of having a single channel filter with a response of .+-.10 kHz, and switching between these filters may cause problems of, for example intermodulation. Furthermore, such additional channel filters are undesirable because channel filters are regarded as being an expensive part of the receiver circuitry.
An alternative area coverage system which operates within a 12.5 kHz channel is termed a quasi-synchronous system in which several transmitters, for example three transmitters, operate at different but very closely related frequencies, the differences being less than 10 Hz. While the system works in principle, there are subjective problems due to the very low frequency beating of the transmitter carrier waves and the very high frequency stability requirements of these transmitters relative to one another.